The next generation communication systems, including 5G, are expected to support high flexibility and a diverse range of services, unlike the previous standards. The IMT-2020 vision defines the use cases into three main categories including, enhanced mobile broadband (eMBB), massive machine type communications (mMTC), and ultrareliable low-latency communications (URLLC) featuring 20 Gb/s peak data rate, 106/km2 device density, and less than 1 ms latency, respectively. The applications which require larger bandwidth and spectral efficiency fall into the eMBB category, whereas the applications that have a tight requirement for device battery life fall into the mMTC category. Typically, industrial smart sensors and medical implants need to be capable of operating for several years without maintenance, and hence, low device complexity and high energy efficiency are crucial for these mMTC services. Furthermore, the mission-critical applications such as remote surgery or self-driving vehicles are represented in the URLLC category. Therefore, a flexible air interface is required to meet these different requirements.
Orthogonal frequency-division multiplexing (OFDM) is the most popular multi-carrier modulation scheme which is currently being deployed in many standards such as 4G LTE and the IEEE 802.11 family. A major disadvantage of OFDM systems is their high out-of-band emissions (OOBE). The OFDM signal is well localized in the time domain with a rectangular pulse shape, which corresponds to a sinc shape in the frequency domain. The sidelobes of the sincs cause significant OOBE and should be reduced to avoid adjacent channel interference (ACI). In particular, the frequency localization is important to allow asynchronous transmission across adjacent sub-bands and to allow coexistence with other waveforms/numerologies in the network. However, a signal cannot be limited in both domains simultaneously, in accordance with the Heisenberg's uncertainty principle. As such, a better spectrum confinement is realized with the cost of expansion in the time domain.
Commonly, OOBE is reduced by various windowing/filtering approaches and numerous waveforms are proposed for the upcoming 5G standard to provide better time-frequency concentration, with certain trade-offs. These known filtering and windowing operations require an additional period which extends the guard duration between consecutive OFDM symbols. Also, extra guard bands are needed in between adjacent channels to control the adjacent channel interference (ACI) along with the windowing/filtering that handles the OOBE. The forthcoming generations must optimize the guards in both time and frequency domains to boost the spectral efficiency.
Accordingly, what is needed in the art is an improved system and method for reducing the out-of-band emissions (OOBE) of the subcarriers (users) in a OFDM-based communication system.